This invention relates to an apparatus for correcting a distance error such as discordance between the present position and the point indicated on a map to represent the present position in a system for performing navigation along a predetermined course.
A method of computing the present position during the operation of a veicle navigation system is known which makes use of a distance sensor for detecting the distance through which the vehicle has traveled, an orientation sensor such as an earth magnetism sensor or gyroscope for detecting the direction of traveling of the vehicle and a map memory unit having a video disk, video tape, CD-ROM or the like. In this method, an image of a map of a particular region is displayed on the basis of corresponding map data read from the map memory unit when required, and the travel locus and the present position of the vehicle are displayed on the display screen, thereby enabling the driver to confirm with the eye the indicated position and ascertain the present position. If there in an error in the indication of the present position, the driver corrects the position of the indication point by moving the same with a cursor or the like.
In such a system of displaying the predetermined travel course and the travel locus, however, the accuracy with which the present position is detected is not sufficient because there is a possibility of occurrence of an error in the detection of orientation if the the earth sensor is used to detect earth magnetism in a particular area where the degree of magnetic disturbance is high, for example, a place near a railroad crossing, a railroad or a power cable. Also, such an error may be caused by magnetization of the vehicle body. The above system is also defective in that the cost of the gyroscope is high and that the method of correcting the present position on the basis of comparison between the calculated positional value and the map data requires a large quantity of data, resulting in an increase in the time taken for computation.
In the method of effecting correction by moving the indication point with a cursor or the like, the positioning is troublesome and cannot easily be performed with desired accuracy.
In consideration of these problems, the applicant of the present invention has proposed in Japanese Patent Application No. 62-333046 a navigation system in which passage through each of intersections on the course is detected to enable distance correction to be performed at each intersection with improved accuracy without influence of disturbance. However, if the interval between intersections where the vehicle turns is large, it is not possible to perform distance correction, resulting in an increase in the accumulated error.